To my knowledge, implementing AES in GPU is only interesting if large data blocks are used (around 30 MB). Also, when counting the cost of memory read and write operations, the gain compared to a CPU implementation that uses assembly drops significantly (

Moreover, if we use AES-NI CPU instructions (which is available on most new CPUs), then a GPU implementation will always be slower then CPU one.

Because if this and knowing how disk encryption uses AES cryptography routines, I can confidently say that GPU will not offer any gain for encryption/decryption speed. The obvious cause is that VeraCrypt supports AES-NI instructions and also the number of memory
read/write operations and the small size of blocks involved.